The present invention relates to novel adjuvant compounds, processes for their preparation, compositions containing them, and their use as vaccine adjuvants.
The refinement and simplification of microbial vaccines and the use of synthetic and recombinant subunit antigens to improve vaccine manufacturability and safety has resulted in a decrease in vaccine potency. This has led to studies on the co-administration of adjuvants with antigens to potentiate vaccine activity and the weak immunogenicity of synthetic and recombinant epitopes. Adjuvants are additives that enhance humoral and/or cell-mediated immune responses to a vaccine antigen. The design of vaccine adjuvants, however, has historically been difficult because of the complex nature of the molecular mechanisms involved in immune system function. Although the addition of microbial components has long been known to enhance adaptive immune responses, only recently was it shown that toll-like receptors (TLRs) on cells involved in immune surveillance, such as epithelial and dendritic cells, engage many of these microbial products via so-called “pathogen-associated patterns” or PAMPs. Many vaccine adjuvants and stand-alone immunomodulators appear to interact with members of the TLR family.
Of the 10 known TLRs that have been identified in humans, five are associated with the recognition of bacterial components (TLRs 1, 2, 4, 5, 6) and four others (TLRs 3, 7, 8, 9) appear to be restricted to cytoplasmic compartments and are involved in the detection of viral RNA (TLRs 3, 7, 8) and unmethylated DNA (TLR9) (Iwasaki, A., Nat Immunol 2004, 5, 987) Activation of TLRs regulates intracellular signaling pathways and leads to gene expression via interaction with intracellular adapter molecules such as MyD88, TRIF, TIRAP, and TRAM (Akira, S. Nat Rev Immunol 2004, 4, 499; Takeda, K. Semin Immunol 2004, 16, 3). These adapter molecules can differentially regulate the expression of inflammatory cytokines/chemokines and type I interferons (IFNα/β), which can lead to the preferential enhancement of antigen-specific humoral and cell-mediated immune responses (Zughaier, S. Infect Immun 2005, 73, 2940). Humoral immunity is the major line of defense against bacterial pathogens, whereas the induction of cytotoxic T lymphocytes (CTLs) appears to be crucial for protective immunity in the case of viral disease and cancer.
In the case of TLR7 and TLR8 activation, a few different classes of small molecule mimetics of the natural (U- and/or G-rich) viral ssRNA ligands have been identified. These include certain antiviral compounds related to oxidized guanosine metabolites (oxoguanosines), which primarily interact with TLR7 (Heil, F. Eur J Immunol 2003, 33, 2987; Hemmi, 2002) and derivatives of adenine which engage TLR7 and/or TLR8. The immune stimulating ability of these compounds has been attributed to the TLR/MyD88-dependent signaling pathways and the production of cytokines, including IL-6 and type I (particularly interferon-α) and II interferons. TLR7 or TLR8 activation leads to the upregulation of co-stimulatory molecules (e.g. CD-40, CD-80, CD-86) and class I and II MHC molecules on dendritic cells (DCs). DCs are the principal cells of the immune system involved in uptake and presentation of antigens to T lymphocytes. Plasmacytoid dendritic cells (pDCs), which preferentially express TLR7, are professional interferon-α producing cells; whereas mDCs express TLR8 only. TLR8 activation on mDCs leads to the preferential production of pro-inflammatory cytokines such as IL-12, TNF-α, and IFN-γ and cell-mediated immunity (CMI). It has been shown that TLR7 agonists are more effective at generating IFN-α and INF-regulated cytokines, whereas TLR8 agonists, which lead to the reversal of CD4+ regulatory (Treg) cell function, are more effective at inducing proinflammatory cytokines such as TNF-α and IL-12, suggesting that TLR7 activation may be more important for antibody responses (Th2-type responses) while TLR8 activation should induce CMI or Th1-type immune responses (Gordon J Immunol 2005, 1259).
One class of TLR-active adenine derivatives that has received a considerable amount of attention are the oxoadenines. The oxoadenines typically contain a hydroxyl group in the 8-position of the adenine ring (often shown in 8-keto/oxo tautomeric form), various substituents at the 2- and 9-positions, and an unsubstituted aromatic amino group in the 4-position. As with other IFN-inducing adenine derivatives such as the imidazoquinolines, an unsubstituted aromatic amino group is thought to be essential for IFN-inducing activity. Many of the oxoadenines, which were initially developed to overcome certain side effects associated with the imidazoquinolines, have been shown to be considerably more potent than prototypical imidazoquinolines such as imiquimod and resiquimod with respect to IFN-inducing activity in vitro and in vivo but devoid of emetic activity, a major clinical side effect with imidazoquinolines. For example, the oxoadenine SM360320 is currently under preclinical development against HCV and has been shown to suppress the replication of HCV in human hepatocytes by type I IFN induction as well as through an IFN-independent mechanism (Lee PNAS 2006, 103, 1828). Nevertheless, despite the fact that the oxoadenine class appears to exhibit better overall toxicity/bioactivity profiles than imidazoquinolines, administration of SM360320 to mice by various routes leads to systemic sickness response, mediated in part by inflammatory cytokines released by mast cells (Hayashi Am J Physiol Regul Integr Comp Physiol 2008, 295, R123). In fact, the large immunological “footprint” of TLR7 agonists in general has led to concerns over toxicity and to suspension of clinical trials in many cases (Strominger Brain Res Bull 2001, 55, 445; Schmidt Nat Biotech 2007, 25, 825).
Since most of the TLR7/8 agonists currently under development often display toxic properties, are unstable, and/or have insubstantial immunostimulatory effects, the discovery and development of effective and safe vaccine adjuvants that activate TLR7 and/or TLR8 is essential for improving the safety and efficacy of existing and new vaccines.